Reorganization of neuronal connections following CNS trauma: principles and experimental paradigms

In Vivo Imaging of the Structural Plasticity of Cortical Neurons After Stroke

The comprehension of the finest mechanisms underlying experience-dependent plasticity requires the investigation of neurons and synaptic terminals in the intact brain over prolonged periods of time. Longitudinal two-photon imaging together with the expression of fluorescent proteins enables high-resolution imaging of dendritic spines and axonal varicosities of cortical neurons in vivo. Importantly, the study of the mechanisms of structural reorganization is relevant for a deeper understanding of the pathophysiological mechanisms of neurological diseases such as stroke and for the development of new therapeutic approaches. This protocol describes the principal steps for in vivo investigation of neuronal plasticity both in healthy conditions and after an ischemic lesion. First, we give a description of the surgery to perform a stable cranial window that allows optical access to the mouse brain cortex. Then we explain how to perform longitudinal two-photon imaging of dendrites, axonal branches, and synaptic terminals in the mouse brain cortex in vivo, in order to investigate the plasticity of synaptic terminals and orientation of neuronal processes. Finally, we describe how to induce an ischemic lesion in a target region of the mouse brain cortex through a cranial window by applying the photothrombotic stroke model.

Matrix Metalloproteinase 9 and Osteopontin Interact to Support Synaptogenesis in the Olfactory Bulb after Mild Traumatic Brain Injury

Olfactory receptor axons reinnervate the olfactory bulb (OB) after chemical or transection lesion. Diffuse brain injury damages the same axons, but the time course and regulators of OB reinnervation are unknown. Gelatinases (matrix metalloproteinase [MMP]2, MMP9) and their substrate osteopontin (OPN) are candidate mediators of synaptogenesis after central nervous system (CNS) insult, including olfactory axon damage. Here, we examined the time course of MMP9, OPN, and OPN receptor CD44 response to diffuse OB injury. FVBV/NJ mice received mild midline fluid percussion insult (mFPI), after which MMP9 activity and both OPN and CD44 protein expression were measured. Diffuse mFPI induced time-dependent increase in OB MMP9 activity and elevated the cell signaling 48-kD OPN fragment. This response was bimodal at 1 and 7 days post-injury. MMP9 activity was also correlated with 7-day reduction in a second 32-kD OPN peptide. CD44 increase peaked at 3 days, delayed relative to MMP9/OPN response. MMP9 and OPN immunohistochemistry suggested that deafferented tufted and mitral neurons were the principal sites for these molecular interactions. Analysis of injured MMP9 knockout (KO) mice showed that 48-kD OPN production was dependent on OB MMP9 activity, but with no KO effect on CD44 induction. Olfactory marker protein (OMP), used to identify injured olfactory axons, revealed persistent axon damage in the absence of MMP9. MMP9 KO ultrastructure at 21 days post-injury indicated that persistent OMP reduction was paired with delayed removal of degenerated axons. These results provide evidence that diffuse, concussive brain trauma induces a post-injury interaction between MMP9, OPN, and CD44, which mediates synaptic plasticity and reinnervation within the OB.

Upper extremity motor training of a subject with initially motor complete chronic high tetraplegia using constraint-induced biofeedback therapy

Introduction

The purpose of this case study was to determine if a subject with chronic high tetraplegia (C3 AIS A) could learn to use an initially paralyzed upper extremity on the basis of training procedures alone.

Case presentation

Initially, an AIS examination revealed no purposive movement below the neck other than minimal shoulder movement. Training was carried out weekly over 39 months. Training began based on electromyographic biofeedback; the electrical activity of a muscle (biceps or triceps) was displayed visually on a computer monitor and the subject was encouraged to progressively increase the magnitude of the response in small increments on a trial-by-trial basis (i.e., shaping). When small, overt movements began to appear; these were, in turn, shaped so that their excursion progressively increased. Training then progressed to enable lifting the arm with the aid of the counterweight of a Swedish Help Arm. Mean movement excursions in the best session were: internal rotation 52.5 cm; external rotation 26.9 cm; shoulder extension 22.1 cm; shoulder flexion 15.2 cm; pronation/supination 120°; extension of index finger (D2) 2.5 cm. Movements were initially saltatory, becoming smoother over time. With the Swedish Help Arm, the subject was able to lift her hand an average of 24.3 cm in the best session with 0.7 kg counterweight acting at the wrist (1.9 J of work).

Discussion

Results suggest in preliminary fashion the effectiveness of this approach for improving upper extremity function after motor complete high tetraplegia. Thus, future studies are warranted. Possible mechanisms are discussed.

Understanding the Mechanisms of Recovery and/or Compensation following Injury

Injury due to stroke and traumatic brain injury result in significant long-term effects upon behavioral functioning. One central question to rehabilitation research is whether the nature of behavioral improvement observed is due to recovery or the development of compensatory mechanisms. The nature of functional improvement can be viewed from the perspective of behavioral changes or changes in neuroanatomical plasticity that follows. Research suggests that these changes correspond to each other in a bidirectional manner. Mechanisms surrounding phenomena like neural plasticity may offer an opportunity to explain how variables such as experience can impact improvement and influence the definition of recovery. What is more, the intensity of the rehabilitative experiences may influence the ability to recover function and support functional improvement of behavior. All of this impacts how researchers, clinicians, and medical professionals utilize rehabilitation.

Array Focal Cortical Stimulation Enhances Motor Function Recovery and Brain Remodeling in a Rat Model of Ischemia

OBJECTIVE

Using a new microelectrode array implanted into the cranial window employing a new stimulation protocol, we investigated the effects of the implanted electrode arrays on both motor map plasticity and neural regeneration in a rodent model of stroke.

MATERIALS AND METHODS

Rats were pretrained on single-pellet retrieval task, then received focal ischemic infarction and microelectrode arrays implantation. Rats in the cortical stimulation (CS) group received daily electrical stimulation (1 hour each day) for 14 days whereas animals in the no stimulation (NS) group did not receive electrical stimulation and only underwent motor mapping. Behavior data and residual electrophysiological mapping on stimulation days 2, 5, 8, 11, and 14 were statistically compared. Neural reorganization in pathological with glial fibrillary acidic protein and microtubule-associated protein-2 was performed.

RESULTS

Rats in CS group showed greater increases in reaching accuracy and significantly decreased in motor threshold than rats in NS group. Immunohistochemical study has shown that array focal CS suppressed inflammatory response, and enhanced dendritic sprouting in the peri-infarction cortex.

CONCLUSION

The present findings support the viability of epidural CS with microelectrode arrays for enhancing motor function after stroke and monitoring the neural reorganization of residual electrophysiological mapping after motor cortex injury.

Axonal Sprouting and Neuronal Connectivity following Central Nervous System Insult: Implications for Occupational Therapy

Based on selected contemporary research, this paper presents a critical analysis of central nervous system (CNS) reorganisation following insult and the need for therapists better to understand the processes that constitute reorganisation and their possible contribution to the development of spasticity. In the treatment of the sequelae of CNS lesions, the synaptic reorganisation as a result of losses caused by injury - in the form of axonal sprouting - is illustrated, focusing on neuronal reconnectivity. Critical analysis of laboratory, electron microscopy and other animal and human studies is also conducted to integrate the controversies identified and to highlight the concepts that become relevant for occupational therapists, in order to optimise therapeutic intervention for maximising restitution in patients with CNS insult. The paper further discusses the capacity of the CNS to compensate and the need to utilise occupational therapy interventions, such as imagining, mental rehearsals, constraint-induced therapy, virtual reality, biofeedback and the traditional repetitive tasks, which leads to ensuring and facilitating the emergence of new synapses to perform motor tasks and manual skills and to prevent secondary changes. These external stimulations provided by the therapists are likely to stimulate both the damaged hemisphere cross-innervation and/or collateral sprouting. These scientifically based treatment strategies and neurological rehabilitation programmes would, in turn, contribute to improving the quality of life of people with CNS insult.

Rewiring neuronal microcircuits of the brain via spine head protrusions--a role for synaptopodin and intracellular calcium stores

Neurological diseases associated with neuronal death are also accompanied by axonal denervation of connected brain regions. In these areas, denervation leads to a decrease in afferent drive, which may in turn trigger active central nervous system (CNS) circuitry rearrangement. This rewiring process is important therapeutically, since it can partially recover functions and can be further enhanced using modern rehabilitation strategies. Nevertheless, the cellular mechanisms of brain rewiring are not fully understood. We recently reported a mechanism by which neurons remodel their local connectivity under conditions of network-perturbance: hippocampal pyramidal cells can extend spine head protrusions (SHPs), which reach out toward neighboring terminals and form new synapses. Since this form of activity-dependent rewiring is observed only on some spines, we investigated the required conditions. We speculated, that the actin-associated protein synaptopodin, which is involved in several synaptic plasticity mechanisms, could play a role in the formation and/or stabilization of SHPs. Using hippocampal slice cultures, we found that ~70 % of spines with protrusions in CA1 pyramidal neurons contained synaptopodin. Analysis of synaptopodin-deficient neurons revealed that synaptopodin is required for the stability but not the formation of SHPs. The effects of synaptopodin could be linked to its role in Ca²⁺ homeostasis, since spines with protrusions often contained ryanodine receptors and synaptopodin. Furthermore, disrupting Ca²⁺ signaling shortened protrusion lifetime. By transgenically reintroducing synaptopodin on a synaptopodin-deficient background, SHP stability could be rescued. Overall, we show that synaptopodin increases the stability of SHPs, and could potentially modulate the rewiring of microcircuitries by making synaptic reorganization more efficient.

Interlimb Reflexes Induced by Electrical Stimulation of Cutaneous Nerves after Spinal Cord Injury

Whether interlimb reflexes emerge only after a severe insult to the human spinal cord is controversial. Here the aim was to examine interlimb reflexes at rest in participants with chronic (>1 year) spinal cord injury (SCI, n = 17) and able-bodied control participants (n = 5). Cutaneous reflexes were evoked by delivering up to 30 trains of stimuli to either the superficial peroneal nerve on the dorsum of the foot or the radial nerve at the wrist (5 pulses, 300 Hz, approximately every 30 s). Participants were instructed to relax the test muscles prior to the delivery of the stimuli. Electromyographic activity was recorded bilaterally in proximal and distal arm and leg muscles. Superficial peroneal nerve stimulation evoked interlimb reflexes in ipsilateral and contralateral arm and contralateral leg muscles of SCI and control participants. Radial nerve stimulation evoked interlimb reflexes in the ipsilateral leg and contralateral arm muscles of control and SCI participants but only contralateral leg muscles of control participants. Interlimb reflexes evoked by superficial peroneal nerve stimulation were longer in latency and duration, and larger in magnitude in SCI participants. Interlimb reflex properties were similar for both SCI and control groups for radial nerve stimulation. Ascending interlimb reflexes tended to occur with a higher incidence in participants with SCI, while descending interlimb reflexes occurred with a higher incidence in able-bodied participants. However, the overall incidence of interlimb reflexes in SCI and neurologically intact participants was similar which suggests that the neural circuitry underlying these reflexes does not necessarily develop after central nervous system injury.

Motor System Reorganization After Stroke: Stimulating and Training Toward Perfection

Stroke instigates regenerative responses that reorganize connectivity patterns among surviving neurons. The new connectivity patterns can be suboptimal for behavioral function. This review summarizes current knowledge on post-stroke motor system reorganization and emerging strategies for shaping it with manipulations of behavior and cortical activity to improve functional outcome.

Time dependent integration of matrix metalloproteinases and their targeted substrates directs axonal sprouting and synaptogenesis following central nervous system injury

Over the past two decades, many investigators have reported how extracellular matrix molecules act to regulate neuroplasticity. The majority of these studies involve proteins which are targets of matrix metalloproteinases. Importantly, these enzyme/substrate interactions can regulate degenerative and regenerative phases of synaptic plasticity, directing axonal and dendritic reorganization after brain insult. The present review first summarizes literature support for the prominent role of matrix metalloproteinases during neuroregeneration, followed by a discussion of data contrasting adaptive and maladaptive neuroplasticity that reveals time-dependent metalloproteinase/substrate regulation of postinjury synaptic recovery. The potential for these enzymes to serve as therapeutic targets for enhanced neuroplasticity after brain injury is illustrated with experiments demonstrating that metalloproteinase inhibitors can alter adaptive and maladaptive outcome. Finally, the complexity of metalloproteinase role in reactive synaptogenesis is revealed in new studies showing how these enzymes interact with immune molecules to mediate cellular response in the local regenerative environment, and are regulated by novel binding partners in the brain extracellular matrix. Together, these different examples show the complexity with which metalloproteinases are integrated into the process of neuroregeneration, and point to a promising new angle for future studies exploring how to facilitate brain plasticity.

Osteopontin expression in acute immune response mediates hippocampal synaptogenesis and adaptive outcome following cortical brain injury

Traumatic brain injury (TBI) produces axotomy, deafferentation and reactive synaptogenesis. Inflammation influences synaptic repair, and the novel brain cytokine osteopontin (OPN) has potential to support axon regeneration through exposure of its integrin receptor binding sites. This study explored whether OPN secretion and proteolysis by matrix metalloproteinases (MMPs) mediate the initial degenerative phase of synaptogenesis, targeting reactive neuroglia to affect successful repair. Adult rats received unilateral entorhinal cortex lesion (UEC) modeling adaptive synaptic plasticity. Over the first week postinjury, hippocampal OPN protein and mRNA were assayed and histology was performed. At 1-2d, OPN protein increased up to 51 fold, and was localized within activated, mobilized glia. OPN transcript also increased over 50 fold, predominantly within reactive microglia. OPN fragments known to be derived from MMP proteolysis were elevated at 1d, consistent with prior reports of UEC glial activation and enzyme production. Postinjury minocycline immunosuppression attenuated MMP-9 gelatinase activity, which was correlated with the reduction of neutrophil gelatinase-associated lipocalin (LCN2) expression, and reduced OPN fragment generation. The antibiotic also attenuated removal of synapsin-1 positive axons from the deafferented zone. OPN KO mice subjected to UEC had similar reduction of hippocampal MMP-9 activity, as well as lower synapsin-1 breakdown over the deafferented zone. MAP1B and N-cadherin, surrogates of cytoarchitecture and synaptic adhesion, were not affected. OPN KO mice with UEC exhibited time dependent cognitive deficits during the synaptogenic phase of recovery. This study demonstrates that OPN can mediate immune response during TBI synaptic repair, positively influencing synapse reorganization and functional recovery.

Noninvasive brain stimulation in traumatic brain injury

OBJECTIVE

To review novel techniques of noninvasive brain stimulation (NBS), which may have value in assessment and treatment of traumatic brain injury (TBI).

METHODS

Review of the following techniques: transcranial magnetic stimulation, transcranial direct current stimulation, low-level laser therapy, and transcranial Doppler sonography. Furthermore, we provide a brief overview of TMS studies to date.

MAIN FINDINGS

We describe the rationale for the use of these techniques in TBI, discuss their possible mechanisms of action, and raise a number of considerations relevant to translation of these methods to clinical use. Depending on the stimulation parameters, NBS may enable suppression of the acute glutamatergic hyperexcitability following TBI and/or counter the excessive GABAergic effects in the subacute stage. In the chronic stage, brain stimulation coupled to rehabilitation may enhance behavioral recovery, learning of new skills, and cortical plasticity. Correlative animal models and comprehensive safety trials seem critical to establish the use of these modalities in TBI.

CONCLUSIONS

Different forms of NBS techniques harbor the promise of diagnostic and therapeutic utility, particularly to guide processes of cortical reorganization and enable functional restoration in TBI. Future lines of safety research and well-designed clinical trials in TBI are warranted to determine the capability of NBS to promote recovery and minimize disability.

Matrix metalloproteinase-3 in the central nervous system: a look on the bright side

Matrix metalloproteinases (MMPs) are a large family of proteases involved in many cell-matrix and cell-cell signalling processes through activation, inactivation or release of extracellular matrix (ECM) and non-ECM molecules, such as growth factors and receptors. Uncontrolled MMP activities underlie the pathophysiology of many disorders. Also matrix metalloproteinase-3 (MMP-3) or stromelysin-1 contributes to several pathologies, such as cancer, asthma and rheumatoid arthritis, and has also been associated with neurodegenerative diseases like Alzheimer's disease, Parkinson's disease and multiple sclerosis. However, based on defined MMP spatiotemporal expression patterns, the identification of novel candidate molecular targets and in vitro and in vivo studies, a beneficial role for MMPs in CNS physiology and recovery is emerging. The main purpose of this review is to shed light on the recently identified roles of MMP-3 in normal brain development and in plasticity and regeneration after CNS injury and disease. As such, MMP-3 is correlated with neuronal migration and neurite outgrowth and guidance in the developing CNS and contributes to synaptic plasticity and learning in the adult CNS. Moreover, a strict spatiotemporal MMP-3 up-regulation in the injured or diseased CNS might support remyelination and neuroprotection, as well as genesis and migration of stem cells in the damaged brain.

Neurodegeneration in the somatosensory cortex after experimental diffuse brain injury

Disruption and consequent reorganization of central nervous system circuits following traumatic brain injury may manifest as functional deficits and behavioral morbidities. We previously reported axotomy and neuronal atrophy in the ventral basal (VB) complex of the thalamus, without gross degeneration after experimental diffuse brain injury in adult rats. Pathology in VB coincided with the development of late-onset aberrant behavioral responses to whisker stimulation, which lead to the current hypothesis that neurodegeneration after experimental diffuse brain injury includes the primary somatosensory barrel cortex (S1BF), which receives projection of VB neurons and mediates whisker somatosensation. Over 28 days after midline fluid percussion brain injury, argyrophilic reaction product within superficial layers and layer IV barrels at 1 day progresses into the cortex to subcortical white matter by 7 days, and selective inter-barrel septa and subcortical white matter labeling at 28 days. Cellular consequences were determined by stereological estimates of neuronal nuclear volumes and number. In all cortical layers, neuronal nuclear volumes significantly atrophied by 42-49% at 7 days compared to sham, which marginally attenuated by 28 days. Concomitantly, the number of healthy neurons was reduced by 34-45% at 7 days compared to sham, returning to control levels by 28 days. Progressive neurodegeneration, including argyrophilic reaction product and neuronal nuclear atrophy, indicates injury-induced damage and reorganization of the reciprocal thalamocortical projections that mediate whisker somatosensation. The rodent whisker barrel circuit may serve as a discrete model to evaluate the causes and consequences of circuit reorganization after diffuse brain injury.

Functional integration of new neurons into hippocampal networks and poststroke comorbidities following neonatal stroke in mice

Stroke in the developing brain is an important cause of chronic neurological morbidities including neurobehavioral dysfunction and epilepsy. Here, we describe a mouse model of neonatal stroke resulting from unilateral carotid ligation that results in acute seizures, long-term hyperactivity, spontaneous lateralized circling behavior, impaired cognitive function, and epilepsy. Exploration-dependent induction of the immediate early gene Arc (activity-regulated cytoskeleton associated protein) in hippocampal neurons was examined in the general population of neurons versus neurons that were generated approximately 1 week after the ischemic insult and labeled with bromodeoxyuridine. Although Arc was inducible in a network-specific manner after severe neonatal stroke, it was impaired, not only in the ipsilateral injured but also in the contralateral uninjured hippocampi when examined 6 months after the neonatal stroke. Severity of both the stroke injury and the acquired poststroke epilepsy negatively correlated with Arc induction and new neuron integration into functional circuits in the injured hippocampi.

Abnormal corticospinal excitability in traumatic diffuse axonal brain injury

This study aimed to investigate the cortical motor excitability characteristics in diffuse axonal injury (DAI) due to severe traumatic brain injury (TBI). A variety of excitatory and inhibitory transcranial magnetic stimulation (TMS) paradigms were applied to primary motor cortices of 17 patients and 11 healthy controls. The parameters of testing included resting motor threshold (MT), motor evoked potential (MEP) area under the curve, input-output curves, MEP variability, and silent period (SP) duration. The patient group overall revealed a higher MT, smaller MEP areas, and narrower recruitment curves compared to normal controls (p < 0.05). The alterations in excitability were more pronounced with an increase in DAI severity (p < 0.005) and the presence of motor impairment (p < 0.05), while co-existence of focal lesions did not affect the degree of MEP changes. MEP variability was significantly lower in the group with motor impairment only (p < 0.05). The intracortical inhibition, as revealed by SP duration, did not exhibit any significant differences in any of the patient groups. In conclusion, our findings expand the concept that impairment of the excitatory and inhibitory phenomena in the motor cortex does not proceed in parallel and demonstrate distinct patterns of aberrations in TBI. Furthermore, these data suggest that alterations in the corticospinal excitatory mechanisms are determined predominantly by the severity of DAI, and show a significant relationship with clinical motor dysfunction following severe trauma diffusely affecting the motor cortical connections. In severe TBI, motor and functional recovery might be linked to restitution of normal corticospinal mechanisms, indexed by normalization of the cortical excitability parameters.

Agrin expression during synaptogenesis induced by traumatic brain injury

Interaction between extracellular matrix proteins and regulatory proteinases can mediate synaptic integrity. Previously, we documented that matrix metalloproteinase 3 (MMP-3) expression and activity increase following traumatic brain injury (TBI). We now report protein and mRNA analysis of agrin, a MMP-3 substrate, over the time course of trauma-induced synaptogenesis. Agrin expression during the successful synaptic reorganization of unilateral entorhinal cortical lesion (UEC) was compared with expression when normal synaptogenesis fails (combined fluid percussion TBI and bilateral entorhinal lesion [BEC]). We observed that agrin protein was increased in both models at 2 and 7 days postinjury, and immuohistochemical (IHC) co-localization suggested reactive astrocytes contribute to that increase. Agrin formed defined boundaries for sprouting axons along deafferented dendrites in the UEC, but failed to do so after combined insult. Similarly, Western blot analysis revealed greater increase in UEC agrin protein relative to the combined TBI+BEC model. Both models showed increased agrin transcription at 7 days postinjury and mRNA normalization by 15 days. Attenuation of synaptic pathology with the NMDA antagonist MK-801 reduced 7-day UEC agrin transcript to a level not different from unlesioned controls. By contrast, MK-801 in the combined insult failed to significantly change 7-day agrin transcript, mRNA levels remaining elevated over uninjured sham cases. Together, these results suggest that agrin plays an important role in the sprouting phase of reactive synaptogenesis, and that both its expression and distribution are correlated with extent of successful recovery after TBI. Further, when pathogenic conditions which induce synaptic plasticity are reduced, increase in agrin mRNA is attenuated.

Electro-acupuncture induced NGF, BDNF and NT-3 expression in spared L6 dorsal root ganglion in cats subjected to removal of adjacent ganglia

This study evaluated the effect of electro-acupuncture (EA) on the NGF, BDNF and NT-3 expression in spared L6 dorsal root ganglion (DRG) in cats subjected to bilateral removal of L1-L5 and L7-S2 DRG, using immunostaining, in situ hybridization and RT-PCR. The positive products of NGF, NT-3 protein and mRNA in the small and large neurons of spared L6 DRG in EA side increased greatly more than that of control side, while the increased BDNF was only noted in small and medium-sized neurons. RT-PCR demonstrated that the mRNA level for three factors was not influenced by EA in intact DRG, when a significant increase was seen in the spared L6 DRG of EA side. As it has been well known that DRG neurons project to the spinal cord wherein morphological plasticity has been present after DRG removal, the present results might have some bearing to the observed phenomenon.

Matrix metalloproteinase-3 expression profile differentiates adaptive and maladaptive synaptic plasticity induced by traumatic brain injury

The interaction between extracellular matrix (ECM) and regulatory matrix metalloproteinases (MMPs) is important in establishing and maintaining synaptic connectivity. By using fluid percussion traumatic brain injury (TBI) and combined TBI and bilateral entorhinal cortical lesion (TBI + BEC), we previously demonstrated that hippocampal stromelysin-1 (MMP-3) expression and activity increased during synaptic plasticity. We now report a temporal analysis of MMP-3 protein and mRNA response to TBI during both degenerative (2 day) and regenerative (7, 15 day) phases of reactive synaptogenesis. MMP-3 expression during successful synaptic reorganization (following unilateral entorhinal cortical lesion; UEC) was compared with MMP-3 expression when normal synaptogenesis fails (after combined TBI + BEC insult). Increased expression of MMP-3 protein and message was observed in both models at 2 days postinjury, and immuohistochemical (IHC) colocalization suggested that reactive astrocytes contribute to that increase. By 7 days postinjury, model differences in MMP-3 were observed. UEC MMP-3 mRNA was equivalent to control, and MMP-3 protein was reduced within the deafferented region. In contrast, enzyme mRNA remained elevated in the maladaptive TBI + BEC model, accompanied by persistent cellular labeling of MMP-3 protein. At 15 days survival, MMP-3 mRNA was normalized in each model, but enzyme protein remained higher than paired controls. When TBI + BEC recovery was enhanced by the N-methyl-D-aspartate antagonist MK-801, 7-day MMP-3 mRNA was significantly reduced. Similarly, MMP inhibition with FN-439 reduced the persistent spatial learning deficits associated with TBI + BEC insult. These results suggest that MMP-3 might differentially affect the sequential phases of reactive synaptogenesis and exhibit an altered pattern when recovery is perturbed.

Genetic approaches to autonomic dysreflexia

Autonomic dysreflexia is a potentially life-threatening condition in which episodic hypertension occurs after injuries above the mid-thoracic segments of the spinal cord. Despite the seriousness of this condition, little is known of the molecular mechanisms that lead to its development. The completed sequencing of the mouse genome, its dense genetic map, and the large repository of engineered and spontaneous mouse mutants, make the mouse an ideal model organism in which to study the molecular mechanisms underlying autonomic dysreflexia. We subjected two wild-type strains of mice, 129Sv and C57BL/6, and one spontaneous mouse mutant, Wallerian degeneration slow (Wld s), to spinal cord transection and clip-compression injury. We found that the incidence of autonomic dysreflexia is greatly reduced, compared to spinal cord-transected wild-type mice, in Wld s mice after both injury paradigms and in 129Sv and C57BL/6 that have undergone the clip-compression injury. We also found that the amplitude of the dysreflexic response was greater in cord-compressed 129Sv than in C57BL/6 mice. These results implicate axonal degeneration as an important source of signals that trigger the development of autonomic dysreflexia and are discussed in the context of mouse genetics, interstrain differences and possible molecular mechanisms underlying autonomic dysreflexia after spinal cord injury.

Regeneration of descending axon tracts after spinal cord injury

Axons within the adult mammalian central nervous system do not regenerate spontaneously after injury. Upon injury, the balance between growth promoting and growth inhibitory factors in the central nervous system dramatically changes resulting in the absence of regeneration. Axonal responses to injury vary considerably. In central nervous system regeneration studies, the spinal cord has received a lot of attention because of its relatively easy accessibility and its clinical relevance. The present review discusses the axon-tract-specific requirements for regeneration in the rat. This knowledge is very important for the development and optimalization of therapies to repair the injured spinal cord.

Reflex reciprocal facilitation of antagonist muscles in spinal cord injury

STUDY DESIGN

Electromyographic study in complete and incomplete spinal cord injury (SCI).

OBJECTIVE

To examine the changes in the pattern of reciprocal inhibition between agonist and antagonist muscles in SCI.

SETTINGS

Sensory Motor Performance Program, Rehabilitation Institute of Chicago, IL, USA.

METHODS

Tendon taps were delivered manually with an instrumented hammer to the tendons of the tibialis anterior and soleus muscle in positions of full-ankle dorsiflexion and plantarflexion in eight subjects with complete SCI and eight subjects with incomplete SCI. Electromyographic activity (EMG) was recorded from ankle dorsiflexor and plantarflexor muscles. Tapping force was also recorded by a force sensor mounted to the tendon hammer, indicating the stimulus onset. Measures of reflex EMG magnitude and reflex latency were obtained for both agonist and antagonist muscles. The ratio of antagonist to agonist EMG was computed based on normalized EMG.

RESULTS

Substantial reflex responses occurred in both the stretched muscle and in its antagonist. The reflex in antagonist, which we term 'reciprocal facilitation (RF)', was most evident in subjects with incomplete SCI. The magnitude of RF was consistently greater than reflex responses in agonist muscles under all test conditions. The latency of the RF was comparable to that of monosynaptic reflex response.

CONCLUSIONS

Following SCI, reciprocal organization of segmental reflexes at the ankle is often partially or completely suppressed, allowing reflex activation in antagonist muscles to be manifested. Possible mechanisms underlying these changes in neural organization are discussed.

SPONSORSHIP

This study was supported by Spinal Cord Research Foundation, the Paralyzed Veterans of America.

Elevation of hippocampal MMP-3 expression and activity during trauma-induced synaptogenesis

The matrix metalloproteinase (MMP) enzyme family contributes to the regulation of a variety of brain extracellular matrix molecules. In order to assess their role in synaptic plasticity following traumatic brain injury (TBI), we compared expression of stromelysin-1 (MMP-3) protein and mRNA in two rodent models of TBI exhibiting different levels of recovery: adaptive synaptic plasticity following central fluid percussion injury and maladaptive synaptic plasticity generated by combined TBI and bilateral entorhinal cortical lesion (TBI + BEC). We sampled the hippocampus at 7 days postinjury, targeting a selectively vulnerable brain region and a survival interval exhibiting rapid synaptogenesis. We report elevated expression of hippocampal MMP-3 mRNA and protein after TBI. MMP-3 immunohistochemical staining showed increased protein levels relative to sham-injured controls, primarily localized to cell bodies within the deafferented dendritic laminae. Injury-related differences in MMP-3 protein were also observed. TBI alone elevated MMP-3 immunobinding over the stratum lacunosum moleculare (SLM), inner molecular layer and hilus, while TBI + BEC generated more robust increases in MMP-3 reactivity within the deafferented SLM and dentate molecular layer (DML). Double labeling with GFAP confirmed the presence of MMP-3 within reactive astrocytes induced by each injury model. Semi-quantitative RT-PCR revealed that MMP-3 mRNA also increased after each injury, however, the combined insult induced a much greater elevation than fluid percussion alone: 1.9-fold vs. 79%, respectively. In the TBI + BEC model, MMP-3 up-regulation was spatio-temporally correlated with increased enzyme activity, an effect which was attenuated with the neuroprotective compound MK-801. These results show that distinct pathological conditions elicited by TBI can differentially affect MMP-3 expression during reactive synaptic plasticity. Notably, these effects are both transcriptional and translational and are correlated with functionally active enzyme.

Interlimb reflex activity after spinal cord injury in man: strengthening response patterns are consistent with ongoing synaptic plasticity

OBJECTIVE

Previous reports from our laboratory have described short-latency contractions in muscles of the distal upper limb following stimulation of lower limb nerves or skin in persons with injury to the cervical spinal cord. It takes 6 or more months for interlimb reflexes (ILR) to appear following acute spinal cord injury (SCI), suggesting they might be due to new synaptic interconnections between lower limb sensory afferents and motoneurons in the cervical enlargement. In this study, we asked if once formed, the strength of these synaptic connections increased over time, a finding that would be consistent with the above hypothesis.

METHODS

We studied persons with sub-acute and/or chronic cervical SCI. ILR were elicited by brief trains of electrical pulses applied to the skin overlying the tibial nerve at the back of the knee. Responses were quantified based on their presence or absence in different upper limb muscles. We also generated peri-stimulus time histograms for single motor unit response latency, probability, and peak duration. Comparisons of these parameters were made in subjects at sub-acute versus chronic stages post-injury.

RESULTS

In persons with sub-acute SCI, the probability of seeing ILR in a given muscle of the forearm or hand was low at first, but increased substantially over the next 1-2 years. Motor unit responses at this sub-acute stage had a prolonged and variable latency, with a lower absolute response probability, compared to findings from subjects with chronic (i.e. stable) SCI.

CONCLUSIONS

Our findings demonstrate that interlimb reflex activity, once established after SCI, shows signs of strengthening synaptic contacts between afferent and efferent components, consistent with ongoing synaptic plasticity.

SIGNIFICANCE

Neurons within the adult human spinal cord caudal to a lesion site are not static, but appear to be capable of developing novel-yet highly efficacious-synaptic contacts following trauma-induced partial denervation. In this case, such contacts between ascending afferents and cervical motoneurons do not appear to provide any functional benefit to the subject. In fact their presence may limit the regenerative effort of supraspinal pathways which originally innervated these motoneurons, should effort in animal models to promote regeneration across the lesion epicenter be successfully translated to humans with chronic SCI.

Recovery of ambulation after traumatic brain injury

OBJECTIVES

To identify variables that are predictive of independent ambulation after traumatic brain injury (TBI) and to define the time course of recovery.

DESIGN

Retrospective review of consecutive admissions of patients with severe TBI over a 32-month period.

SETTING

Brain injury unit in an acute, inpatient rehabilitation hospital.

PARTICIPANTS

Of 264 patients screened, 116 met criteria that included the ability to participate in motor and functional evaluation on admission to acute rehabilitation, and the absence of other neurologic disorders or fractures that affect one's ability to ambulate.

INTERVENTION

Inpatient rehabilitation on a specialized TBI unit by an interdisciplinary team.Main outcome measures Recovery of independent ambulation and time to recover independent ambulation.

RESULTS

Of eligible patients, 73.3% achieved independent ambulation by latest follow-up (up to 5.1 mo). Patients who achieved independent ambulation were significantly younger (P<.05), had better gait scores on admission (P<.05), and tended to be less severely injured-based on duration of posttraumatic amnesia (PTA; P=.058)-than those who did not ambulate independently. There were no differences in recovery based on neuropathologic profile. Mean time to independent ambulation +/- standard deviation was 5.7+/-4.3 weeks; of those achieving independent ambulation, 82.4% did so by 2 months and 94.1% by 3 months. If not independent by 3 months postinjury, patients had a 13.9% chance of recovery. Multivariate regression analysis generated prediction models for time to independent ambulation, using admission FIM instrument scores and age (38% of variance); initial gait score, loss of consciousness, and age (40% of variance); or initial gait score and PTA (58% of variance), when restricted to just those patients with diffuse axonal injury.

CONCLUSIONS

Most patients with severe TBI achieved independent ambulation; the vast majority did so within 3 months postinjury. Functional measures, injury severity measures, and age can help guide prognosis and expectations for time to recover.

Immediate plasticity in the motor pathways after spinal cord hemisection: implications for transcranial magnetic motor-evoked potentials

The present study evaluates motor functional recovery after C2 spinal cord hemisection with or without contralateral brachial root transection, which causes a condition that is similar to the crossed phrenic phenomenon on rats. Descending motor pathways, including the reticulospinal extrapyramidal tract and corticospinal pyramidal tracts, were evaluated by transcranial magnetic motor-evoked potentials (mMEPs) and direct cortical electrical motor-evoked potentials (eMEP), respectively. All MEPs recorded from the left forelimb were abolished immediately after the left C2 hemisection. Left mMEPs recovered dramatically immediately after contralateral right brachial root transection. Corticospinal eMEPs never recovered, regardless of transection. The facilitation of mMEPs in animals that had undergone combined contralateral root transection was well correlated with open-field behavioral motor performance. Both electrophysiological and neurological facilitations were significantly attenuated by the selective serotonin synthesis inhibitor para-chlorophenylalanine (p-CPA). These results suggest that serotonergic reticulospinal fibers located contralateral to hemisection contribute to the behavioral and electrophysiological improvement that immediately follows spinal cord injury (SCI).

Seven cDNAs enriched following hippocampal lesion: possible roles in neuronal responses to injury

Synaptic plasticity is important for formation of long-term memories and in re-establishment of function following injury. Seven cDNAs enriched following lesion in the hippocampus of the rat have been isolated using a PCR-based cDNA suppression subtraction hybridization. Sequence analysis resulted in the identification of two genes with known roles in synaptic development and neuronal activities: astrotactin and calcineurin. These two neuron-specific genes have established roles in development or synaptogenesis. Sequence analysis of the other five additional genes shows that two are likely to be involved in G-protein signaling pathways, one is a WD repeat protein, and the remaining two are entirely novel. All seven candidates are expressed in the hippocampus and, in some cases, cortical layers of adult brains. RT-PCR data show that expression increases following synaptogenic lesion. Immunocytochemical analysis in primary hippocampal neurons showed that Calcineurin immunoreactivity was redistributed in neurons during 2 weeks in culture. This redistribution suggests that Calcineurin's role changes during neurite outgrowth immediately prior to synapse formation in vitro. In addition, inhibiting Calcineurin activity with cyclosporin A enhanced neurite outgrowth, suggesting that Calcineurin has a regulatory role in axon sprouting. The discovery of previously unknown genes involved in the response to neurodegeneration will contribute to our understanding of neural development, responses to CNS trauma, and neurodegenerative diseases.

CNS regeneration: clinical possibility or basic science fantasy?

Following injury to the CNS, severed axons undergo a phase of abortive sprouting in the vicinity of the wound, but do not spontaneously re-grow or regenerate. From a long history of attempts to stimulate regeneraion, a major strategy that has been developed clinically is the implantation of tissue into denervated target regions. Unfortunately trials have so far not borne out the promise that this would prove a useful therapy for disorders such as Parkinson's disease. Many strategies have also been developed to stimulate the regeneration of axons across sites of injury, particularly in the spinal cord. Animal data have demonstrated that some of these approaches hold promise and that the spinal cord has a remarkable degree of intrinsic plasticity. Attempts are now being made to utilize experimental techniques in spinal patients.

Autonomic dysreflexia after spinal cord transection or compression in 129Sv, C57BL, and Wallerian degeneration slow mutant mice

To study plasticity of central autonomic circuits that develops after spinal cord injury (SCI), we have characterized a mouse model of autonomic dysreflexia. Autonomic dysreflexia is a condition in which episodic hypertension occurs after injuries above the midthoracic segments of the spinal cord. As synaptic plasticity may be triggered by axonal degeneration, we investigated whether autonomic dysreflexia is reduced in mice when axonal degeneration is delayed after SCI. We subjected three strains of mice, Wld(S), C57BL, and 129Sv, to either spinal cord transection (SCT) or severe clip-compression injury (CCI). The Wld(S) mouse is a well-characterized mutant that exhibits delayed Wallerian degeneration. The CCI model is an injury paradigm in which significant the axonal degeneration is due to secondary events and therefore delayed relative to the time of the initial injury. We herein demonstrate that the incidence of autonomic dysreflexia is reduced in Wld(S) mice after SCT and in all mice after CCI. To determine if differences in afferent arbor sprouting could explain our observations, we assessed changes in the afferent arbor in each mouse strain after both SCT and CCI. We show that independent of the type of injury, 129Sv mice but not C57BL or Wld(S) mice demonstrated an increased small-diameter CGRP-immunoreactive afferent arbor after SCI. Our work thus suggests a role for Wallerian degeneration in the development of autonomic dysreflexia and demonstrates that the choice of mouse strain and injury model has important consequences to the generalizations that may be drawn from studies of SCI in mice.

Ascending sensory, but not other long-tract axons, regenerate into the connective tissue matrix that forms at the site of a spinal cord injury in mice

Mice exhibit a unique wound healing response following spinal cord injury in which the lesion site fills in with a connective tissue matrix. Previous studies have revealed that axons grow into this matrix, but the source of the axons remained unknown. The present study assesses whether any of these axons were the result of long tract regeneration. C57Bl/6 mice received crush injuries and were allowed to survive for 6 weeks to 7 months. Biotinylated dextran amine (BDA) was injected into the somato-motor cortex to trace descending corticospinal tract (CST) axons, into the midbrain to label descending brainstem pathways including the rubrospinal and reticulospinal tracts, or into the L5 dorsal root ganglion to trace ascending projections of first-order sensory neurons. Spinal cords from other mice were prepared for immunocytochemistry using antibodies against neurofilament protein (NF), 5-HT to reveal descending serotonergic axons, calcitonin gene-related protein (CGRP) to reveal ascending sensory axons, and chondroitin sulfate proteoglycan (CSPG) to assess the distribution of molecules that are inhibitory to axon growth. NF immunostaining revealed axons in the connective tissue matrix at the lesion site, confirming previous studies that used protargol staining. CST axons did not enter the connective tissue matrix, but did sprout extensively in segments adjacent to the injury site. Rubrospinal and reticulospinal tract axons also did not grow into the lesion site. 5-HT-positive axons extended to the edge of the lesion, and a few axons followed astrocyte processes into the margins of the lesion site. In contrast to the other pathways, BDA-labeled ascending sensory axons did extend into and arborized extensively within the connective tissue matrix, although the subgroup of ascending axons that are positive for CGRP did not. These results indicate that the connective tissue matrix is permissive for regeneration of some classes of ascending sensory axons but not for other axonal systems.

Visualizing changes in circuit activity resulting from denervation and reinnervation using immediate early gene expression

We describe a novel strategy to evaluate circuit function after brain injury that takes advantage of experience-dependent immediate early gene (IEG) expression. When normal rats undergo training or are exposed to a novel environment, there is a strong induction of IEG expression in forebrain regions, including the hippocampus. This gene induction identifies the neurons that are engaged during the experience. Here, we demonstrate that experience-dependent IEG induction is diminished after brain injury in young adult rats (120-200 gm), specifically after unilateral lesions of the entorhinal cortex (EC), and then recovers with a time course consistent with reinnervation. In situ hybridization techniques were used to assess the expression of the activity-regulated cytoskeleton-associated protein Arc at various times after the lesion (4, 8, 12, 16, or 30 d). One group of rats was allowed to explore a complex novel environment for 1 hr; control operated animals remained in their home cage. In unoperated animals, exposure to the novel environment induced Arc mRNA levels in most pyramidal neurons in CA1, in many pyramidal neurons in CA3, and in a small number of dentate granule cells. This characteristic pattern of induction was absent at early time points after unilateral EC lesions (4 and 8 d) but recovered progressively at later time points. The recovery of Arc expression occurred with approximately the same time course as the reinnervation of the dentate gyrus as a result of postlesion sprouting. These results document a novel approach for quantitatively assessing activity-regulated gene expression in polysynaptic circuits after trauma.

Identification of neuronal cell death in a model of degeneration in the hippocampus

Neuronal cell death and microglial changes are both hallmarks of neurodegenerative disorders. Therefore, analysis of degenerating neurons related to microglial changes are addressed in many studies of neurosciences. Here we compared different lesion models and two markers for neurodegeneration (Fluoro-Jade and propidium iodide) in an in vivo as well as an in vitro approach. Fluoro-Jade is a specific and selective marker to identify neurons undergoing degeneration. We also tested this marker to analyze neurodegeneration in organotypic hippocampal slice cultures. We could show that activation of microglia is followed by neuronal cell death. Most degeneration markers, such as propidium iodide, only stain the neuronal cell body excluding the axonal and dendritic processes. Fluoro-Jade is able to stain the distal portion as well as the proximal portion of the dissected axon including the axotomized neuron, as so called anterograde and retrograde degeneration after axotomy. To analyze the specificity of Fluoro-Jade, we used primary microglial and BV-2 cells, a well-described murine microglial cell line. Treatment of microglial and BV-2 cells with an excess of L-glutamate induces cell death which could be detected by propidium iodide staining, but not by Fluoro-Jade, demonstrating its specificity to monitor neuronal cell death.

Autoimmunity as a special case of immunity: removing threats from within

The function of the adaptive immune response against exogenous (non-self) agents is to help the innate arm of the immune system (represented by phagocytic cells) to fight and eliminate these agents. We suggest that the body also protects itself against potentially harmful self components using mechanisms similar to those used for fighting and eliminating non-self agents, and that the protective immune activity against self-components competes with the activity of self-destructive compounds. Tolerance to self is thus not a lack of response to self, but the ability to tolerate an active defense response to self without developing an autoimmune disease.

Effects of sustained stimulation on the excitability of motoneurons innervating paralyzed and control muscles

The excitability of thenar motoneurons (reflected by F-wave persistence and amplitude) and thenar muscle force were measured during a stimulation protocol (90 s of 18-Hz supramaximal electrical stimulation of the median nerve) designed to induce muscle fatigue (force decline). Data from muscles (n = 15) paralyzed by chronic cervical spinal cord injury were compared with those obtained from control muscles (n = 6). The persistence of F waves in both paralyzed and control muscles increased from approximately 60 to approximately 76% during the first 10 s of the fatigue protocol. Persistence then declined progressively to approximately 33% at 90 s. These changes in F-wave persistence suggest that similar reductions occur in the excitability of the motoneurons to paralyzed and control motor units after sustained antidromic activation. Despite this, significantly larger force declines occurred in the paralyzed muscles of spinal cord-injured subjects (approximately 60%) than in the muscles of control subjects (approximately 15%). These data suggest that the decreases in motoneuron excitability for both the spinal cord-injured and control subjects are a result of activity-dependent changes in motoneuron properties that are independent of fatigue-related processes in the muscles.

Interlimb reflexes and synaptic plasticity become evident months after human spinal cord injury

Persons with long-standing injury to the cervical spinal cord resulting in complete or partial paralysis typically develop a wide spectrum of involuntary movements in muscles receiving innervation caudal to the level of injury. We have previously shown that these movements include brief and discrete contraction of muscles in the hand and forearm in response to innocuous sensory stimulation to the feet and legs, but we have been unable to replicate these interlimb reflexes in able- bodied subjects. Properties of these muscle responses indicate that the synaptic contacts between ascending sensory fibres and motor neurones of the cervical enlargement are more efficacious than normal. If these connections are present at all times, and require the more rostrally-placed spinal cord injury to allow their emergence, one might expect their appearance relatively soon following injury, as has been shown for studies of 'latent' synapses. Conversely, delayed appearance of these interlimb reflexes would suggest either the development of new synaptic connections or a profound strengthening of existing circuits in the cervical spinal cord due to a combination of afferent target loss and motor neurone denervation from motor tracts originating rostral to the injury site. In this study, we used repeated examinations of persons with acute injury to the cervical spinal cord to examine the time post-injury at which interlimb reflexes are first seen. Using tibial nerve stimulation at the knee as a screening test, a total of 24 subjects were found to develop interlimb reflexes following spinal cord injury. Latencies between stimulation and EMG were as brief as 32 ms for muscles of the forearm and 44 ms for muscles in the hand. These minimal delays all but rule out a supraspinal route for these interlimb reflexes. Interlimb reflexes first became evident no sooner than approximately 6 months following injury, and in some individuals were not seen until well over 1 year post-injury. Enhanced lower limb segmental excitability had emerged in nearly all of these subjects weeks or months prior to the first appearance of interlimb reflexes, arguing against a manifestation of traditional post-traumatic spasticity as a basis for this activity. This prolonged delay between time of injury and emergence of interlimb reflex activity lends support to the hypothesis that this activity represents an example of plasticity-and perhaps 'regenerative sprouting'-in the human spinal cord following traumatic injury.

Molecules involved in reactive sprouting in the hippocampus

Denervation of the hippocampus triggers reactive responses in neurons and glial cells in their affected strata in a temporally ordered fashion. Many of these responses have been studied extensively, focusing on the one hand on glial initiation and clearing responses during the degeneration phase and, on the other, on transneuronal reorganization and the newly adjusted physiological balance. We used the entorhinal cortex lesion (ECL) as a model system to study the cues that underlie the layer-specific sprouting response. This lesion destroys the perforant path, which is a massive excitatory projection to the dentate gyrus and hippocampus proper. In the deafferented zones of the hippocampus, sprouting of the remaining unlesioned fibers occurs, which replaces the lost afferences of the perforant path. We focus on candidate molecules which govern the layer-specific sprouting of the remaining axons and, in particular, on membrane-bound cues. The fact that layer-specific sprouting occurs even in the adult central nervous system (CNS) provides a valuable model for understanding the mechanisms of reactive neuronal growth and reorganization in the adult CNS. Isolation and analysis of the molecules involved in these mechanisms are important steps in understanding the potential and limitations of regeneration in the CNS.

Molecular and functional analysis of hyperpolarization-activated pacemaker channels in the hippocampus after entorhinal cortex lesion

Differential display of hippocampal tissue after entorhinal cortex lesion (ECL) revealed decreases in mRNA encoding the neuronal hyperpolarization-activated, cyclic nucleotide-gated channel HCN1. In situ hybridization confirmed that hippocampal transcripts of HCN1, but not HCN2/3/4, are down-regulated after ECL. Expression recovered at approximately 21 days after lesion (dal). Immunohistochemistry demonstrated a corresponding regulation of HCN1 protein expression in CA1-CA3 dendrites, hilar mossy cells and interneurons, and granule cells. Patch-clamp recordings in the early phase after lesion from mossy cells and hilar interneurons revealed an increase in the fast time constant of current activation and a profound negative shift in voltage activation of Ih. Whereas current activation recovered at 30 dal, the voltage activation remained hyperpolarized in mossy cells and hilar interneurons. Granule cells, however, were devoid of any detectable somatic Ih currents. Hence, denervation of the hippocampus decreases HCN1 and concomitantly the Ih activity in hilar neurons, and the recovery of h-current activation kinetics occurs parallel to postlesion sprouting.

Perforant path lesion induces up-regulation of stathmin messenger RNA, but not SCG10 messenger RNA, in the adult rat hippocampus

In this study, we performed in situ hybridization analysis of the expression pattern of two growth-associated proteins, stathmin and SCG10, in the hippocampus after unilateral lesion of the perforant pathway, the main excitatory input from the entorhinal cortex to the hippocampus. Stathmin is one of the major neural-enriched cytosolic phosphoproteins and a potential target of cyclic-AMP-dependent kinases [Jin L. W. et al. (1996) Neurobiol. Aging 17, 331-341; Leighton I. A. et al. (1993) Molec. Cell Biochem. 127/128, 151-156]. Three days after the lesion, stathmin messenger RNA was up-regulated ipsilaterally in the hilus, in the granule cell layer of the dentate gyrus and in the pyramidal cell layer of the CA1 region. Simultaneously, the hilar region of the contralateral dentate gyrus showed an increased stathmin messenger RNA expression. This altered expression pattern was observed until 15 days after lesion. Stathmin messenger RNA expression returned to a normal level until 21 days after lesion in all regions analysed. SCG10, a membrane-bound neuronal growth-associated protein belonging to the SCG10/stathmin gene family, did not show any alteration of messenger RNA expression after perforant path lesion. The temporal changes of stathmin messenger RNA expression in the ipsilateral hippocampus correspond well to the process of reactive synaptogenesis. The enhanced messenger RNA expression in the hilar region of the contralateral dentate gyrus might suggest a role in neurite elongation, since this region is the origin of commissural fibres involved in the sprouting response in the deafferented hippocampus. The present study provides evidence that the induction of specific growth-associated proteins is differentially regulated in the hippocampus.

Effects of red nucleus ablation and exogenous neurotrophin-3 on corticospinal axon terminal distribution in the adult rat

Collateral sprouting of undamaged descending axons is one potential mechanism for recovery of function after incomplete spinal cord injury. In this study, we have investigated whether terminals of the intact corticospinal tract in the rat would sprout following ablation of a parallel descending pathway, the rubrospinal tract. No sprouting was detected after this injury alone. However, the combination of rubrospinal tract ablation with administration of 100ng neurotrophin-3 to neurons of the corticospinal tract resulted in marked increased density of corticospinal innervation in the superficial dorsal horn. There was no effect of administration of neurotrophin-3 alone and increase in axon density was not detected in the deep dorsal horn. These results imply that spontaneous sprouting of undamaged corticospinal axons does not occur following ablation of a parallel tract system, although collateral sprouting can be induced through a combination of the lesion plus exogenous growth factor. Induced change in corticospinal terminal density is detected in the superficial dorsal horn only, supporting the hypothesis that this is an area particularly supportive of circuit reorganisation.

An alpha(2)-adrenergic antagonist, atipamezole, facilitates behavioral recovery after focal cerebral ischemia in rats

Previous studies suggest that enhanced noradrenergic neurotransmission promotes functional recovery following cerebral lesions. The present study investigated whether systemic administration of an alpha(2)-adrenergic antagonist, atipamezole, facilitates recovery following transient focal cerebral ischemia in rats. The effect of atipamezole therapy on recovery from ischemia was compared with the effect of enriched-environment housing in rats. Ischemia was induced by occlusion of the right middle cerebral artery (MCA) for 120 min using the intraluminal filament model. Daily atipamezole treatment (1 mg/kg, subcutaneously) was started on day 2 after ischemia induction and drug administration stopped after 10 days. Another group of rats was housed in an enriched environment from day 2 following ischemia induction until the end of the experiment. Several different behavioral tests were used to measure functional recovery during the 26 days following the induction of focal cerebral ischemia. There was improved performance in the limb-placing test from the beginning of atipamezole treatment to day 8, and in wheel-running in the foot-slip test on days 2 and 4. Enriched-environment housing facilitated recovery in the foot-slip test in a later phase of the test period (days 8 to 10). Discovery of a hidden platform in a water-maze task was also facilitated in rats housed in the enriched environment, but this was probably due to the increased swimming speed of these rats. The present data suggest that the alpha(2)-adrenergic antagonist, atipamezole, facilitates sensorimotor recovery after focal ischemia, but has no effect on subsequent water-maze tests assessing spatial learning and memory, when assessed 11 days after the cessation of drug administration.

The hippocampus: anatomy, pathophysiology, and regenerative capacity

Cognitive deficits following insults to the central nervous system-particularly those involving the hippocampus and related structures-are often persistent and severely debilitating. Progress has been made in establishing the role of the hippocampus in integrating information in the formation of memory necessary for subsequent recollection of information. The present article will review anatomic, physiological, and functional aspects of the hippocampus in reference to learning and memory. Both animal and human hippocampal pathophysiological processes will be explored. Adaptive and maladaptive central nervous system responses will be reviewed, with a special emphasis on neurogenesis. Ideally, physiological and cellular compensatory responses ought to parallel clinical observation. However, this association is not clearly established. Finally, the current understanding of neuromodulatory mechanisms (although quite preliminary) will also be discussed.

Outgrowth-promoting molecules in the adult hippocampus after perforant path lesion

Lesion-induced neuronal plasticity in the adult central nervous system of higher vertebrates appears to be controlled by region- and layer-specific molecules. In this study we demonstrate that membrane-bound hippocampal outgrowth-promoting molecules, as present during the development of the entorhino-hippocampal system and absent or masked in the adult hippocampus, appear 10 days after transection of the perforant pathway. We used an outgrowth preference assay to analyse the outgrowth preference of axons from postnatal entorhinal explants on alternating membrane lanes obtained from hippocampus deafferented from its entorhinal input taken 4, 10, 20, 30 and 80 days post-lesion and from adult control hippocampus. Neurites from the entorhinal cortex preferred to extend axons on hippocampal membranes disconnected from their entorhinal input for 10 days in comparison with membranes obtained from unlesioned adult animals. Membranes obtained from hippocampi disconnected from their entorhinal input for 10 days were equally as attractive for growing entorhinal cortex (EC) axons as membranes from early postnatal hippocampi. Further analysis of membrane properties in an outgrowth length assay showed that entorhinal axons extended significantly longer on stripes of lesioned hippocampal membranes in comparison with unlesioned hippocampal membranes. This effect was most prominent 10 days after lesion, a time point at which axonal sprouting and reactive synaptogenesis are at their peak. Phospholipase treatment of membranes obtained from unlesioned hippocampi of adult animals strongly promoted the outgrowth length of entorhinal axons on these membranes but did not affect their outgrowth preference for deafferented hippocampal membranes. Our results indicate that membrane-bound outgrowth-promoting molecules are reactivated in the adult hippocampus following transection of the perforant pathway, and that neonatal entorhinal axons are able to respond to these molecules. These findings support the hypothesis of a temporal accessibility of membrane-bound factors governing the layer-specific sprouting of remaining axons following perforant path lesion in vivo.

Spinal cord injury in small animals 2. Current and future options for therapy

Although there can be substantial spontaneous improvements in functional status after a spinal cord injury, therapeutic intervention is desirable in many patients to improve the degree of recovery. At present only decompressive surgery and the neuroprotective drug methylprednisolone sodium succinate are effective and in widespread clinical use. There are limitations to the efficacy of these therapies in some clinical cases and they cannot restore satisfactory functional status to all patients. Many drugs have been investigated experimentally to assess their potential to preserve injured tissue and promote functional recovery in clinically relevant settings, and several of them would be suitable for assessment in future veterinary clinical trials. In addition, experimental techniques designed to mould the response of the CNS to injury, by the promotion of axonal regeneration across the lesion and the encouragement of local sprouting of undamaged axons, have recently been successful, suggesting that effective therapy for even very severe spinal cord injury may soon be available.

Incomplete spinal cord injury: neuronal mechanisms of motor recovery and hyperreflexia

OBJECTIVE

To understand neuronal mechanisms of motor recovery and hyperreflexia after incomplete spinal cord injury (SCI), and their role in rehabilitation.

DESIGN

Reviewed and compared clinical, neurophysiologic, and neuropathologic data from human SCI patients with behavioral, neurophysiologic, and neuroanatomic data from animals to postulate underlying neuronal mechanisms.

OUTCOME

A postulation that two neuronal mechanisms-receptor up-regulation and synapse growth-act sequentially, to explain the gradual appearance of motor recovery after incomplete SCI. These same mechanisms may also act in spinal reflex pathways to mediate hyperreflexia caudal to SCI.

RESULTS

After incomplete SCI, walking ability and hyperreflexia often develop. Initially, cord neurons are hyperpolarized and less excitable because of loss of normal descending facilitation; this is spinal shock. Then, gradually, voluntary movement recovers and hyperreflexia develops. Early (hours to days), these changes develop simultaneously, suggesting a common postsynaptic mechanism-likely, an increase in postsynaptic receptor excitability, possibly receptor up-regulation. Late (weeks to months), recovery and reflex changes occur at a slow rate, are no longer simultaneous, and are long-lasting, which suggests a presynaptic mechanism, such as local synapse growth in spared descending pathways and in reflex pathways. This presumed synapse growth is seemingly enhanced by active use of the growing pathway. Also, developing hyperreflexia appears to limit motor recovery.

CONCLUSIONS

These observations suggest that rehabilitation for incomplete SCI should (1) increase activity in spared descending motor pathways, (2) initially use reflex facilitation or central nervous system stimulants to assist spared descending inputs in depolarizing cord neurons, and (3) later minimize reflex input, when spared descending inputs can depolarize cord neurons without reflex facilitation. Better understanding of neuronal mechanisms that underlie motor recovery after incomplete SCI promises better outcomes from rehabilitation.